The present invention relates to a process for removing acid gases from a water vapour-containing fluid stream, in particular for removing carbon dioxide from combustion exhaust gases.
Combustion of fossil fuels generates the acid gases, carbon dioxide (CO2), sulfur oxides (SO2), carbonyl sulfide (COS), hydrogen sulfide (H2S) and nitrogen oxides (NOx). Emission of acid gases into the air is considered to be the main cause of global effects on the environment which are termed the greenhouse effect. The removal of acid gases from combustion exhaust gases is therefore desirable to reduce the environmental effects of these sources.
Current aqueous scrubbing technologies remove acid gases by treating the fluid streams with aqueous solutions of inorganic or organic bases, e.g. alkanolamines, as absorbents. On the dissolution of acid gases, ionic products form from the base and the acid gas components. The absorbent can be regenerated by heating, expansion to a lower pressure or stripping, in which case the ionic products react back to acid gases and/or the acid gases are stripped off by means of steam. After the regeneration process the absorption liquid can be reused.
Non-aqueous absorption liquids have been used to remove acid gases from fluid streams. It is claimed that non-aqueous absorption liquids require less energy for regeneration than aqueous solutions of organic bases. Single-component alcoholic physisorption solvents such as RECTISOL® and SELEXOL® are commercially available for CO2 separation but perform poorly in the humid, near-ambient pressure conditions associated with combustion exhaust gas.
Ionic liquids are another non-aqueous absorption liquid currently being developed. These absorption liquids have higher CO2 loadings than some aqueous amines, and are regenerable under milder conditions.
WO 2009/097317 discloses reversible acid-gas binding liquid systems that permit capture of acid gases and release of the acid gases from the liquid by the activation of a trigger. The exemplified systems are equimolar mixtures of amidine or guanidine nitrogen bases and alcohols. They are non-ionic room temperature liquids that react with CO2 to form room-temperature ionic liquids. CO2 is captured via the formation of amidinium and guanidinium alkyl carbonate salts derived from the conjugate bases of the deprotonated alcohol components.
Combustion exhaust gases typically contain appreciable amounts of water vapour or are water vapour-saturated. Upon contact of a non-aqueous absorption liquid with the water vapour-containing fluid stream, water vapour may condense and/or become dissolved into the non-aqueous absorption liquid. On the other hand, the non-aqueous absorption liquid may not show a significant water vapour partial pressure, so that the treated fluid stream will not take up an appreciable amount of water vapour when leaving the absorption unit.
These effects can lead to the accumulation of water in the absorption liquid either as a single-phase solution or bi-phase mixture, depending upon the process conditions (e.g., pressure, temperature, water concentration) and the affinity of the non-aqueous absorption liquid for water. Water accumulation is detrimental to the acid gas separation and purification process, since more energy will be required for absorption liquid regeneration due to the necessity of continually removing water from the absorption liquid.
Thus, the amount of water that accumulates in the absorption liquid system must be removed from the process in order to fulfil the overall water balance.
When the absorption liquid is incompletely miscible with water (or, in other words, shows a miscibility gap with water), the water introduced may sooner or later accumulate as a separate aqueous phase within the absorption liquid system. The aqueous phase may be separated by, e.g., a decanter, and discharged as a liquid bleed. Since, however, the absorption liquid shows—even though limited—solubility in water the liquid bleed contains traces of absorption liquid and will cause undesirable absorption liquid losses. To be properly discharged, the liquid bleed must be purified. Purification of the liquid bleed is costly and technically difficult and therefore not a preferred option. As a result, bleeding liquid water constitutes a clear economic and ecological disadvantage and decreases the commercial attractiveness of the process.
Accordingly, there is a need for a process for removing acid gases from a water vapour-containing fluid stream, which allows for an efficient removal of water accumulated in the absorption liquid system.
EP-A 2 228 119 discloses a method for removal of acid compounds from a gas by carrying out an absorption step with an absorption liquid. The treated gas is washed and cooled in a washing section by contacting with a liquid water flow having a predetermined temperature, so as to obtain a washed gas whose temperature is lower than temperature of the gas to be treated. A quantity of water contained in the acid compound-rich effluent is withdrawn to prevent accumulation of water in the absorption liquid.
The invention provides a process for removing acid gases from a water vapour-containing fluid stream comprising                a) providing an absorption liquid which is incompletely miscible with water;        b) treating the fluid stream in an absorption zone with the absorption liquid to obtain an acid gas-depleted treated fluid stream and an acid gas-loaded absorption liquid;        c) directing the treated fluid stream to a rehydration zone and treating the fluid stream with an aqueous liquid to volatilize at least part of the aqueous liquid;        d) regenerating the loaded absorption liquid to expel the acid gases at least in part and obtain a regenerated absorption liquid, and directing the regenerated absorption liquid to step b); and        e) separating, from the absorption liquid, an aqueous liquid that has condensed in the absorption zone, and directing the aqueous liquid to step c).        